1. Field of the Invention
This Invention relates to the field of Gas Turbine Engines, and, more specifically, to an annular tip shroud for a full row of rotating turbine blades.
2. Description of the Related Art
Rotating blades in a gas turbine engine are known to include tip shrouds to provide a gas seal for the flow path through the turbine, to provide a change in vibration response, and to provide vibration damping of the blades. A tip shroud will add mass to the blade, which will increase the pull and airfoil stress of the specific blade. A designer typically will use the tip shroud for a set of blades in a particular row to control the modal frequency of the blade such that the natural frequency of the blade will not coincide with a driving frequency of the gas turbine engine. This matching of frequencies could produce resonant vibration in the blade, which can cause damage to the blade. Also, the size of the shroud will increase the stress in the blade and require the rotor disc to be larger in order to carry the increased load.
The last stage of the turbine has blades with the largest diameters. In order to extract the largest amount of power from the flowing gases, it is preferable to provide the last stage of the turbine with the largest diameter of blades possible. The power extracted from the last stage of the turbine is related to the equation An2, where A is the circumferential area of the rotational plane of the blade diameter and N is the rotational speed of the turbine. The larger the value of An2 the larger the amount of power the last stage of the turbine can extract from the flowing gases.
However, as the blade diameter increases, the centrifugal force acting on the blade increase, this force being larger with the addition of tip shrouds on the blades. Thus, the blade diameter—and, therefore, the power extracted from the flowing gases—is limited to the resulting stresses in which the blade is capable of withstanding.
U.S. Pat. No. 5,037,273 issued to Kreuger et al on Aug. 6, 1991 shows a compressor impeller with a ring shaped shroud band which is mounted at blade tips, each blade tip being enclosed in an identical radially slidable manner by a guide block, and the guide blocks being fastened to the shroud band. In the Kreuger et al invention, the blade tip slides within a closed opening in the shroud ring assembly, and the shroud ring assembly is made up of fiber reinforced and metal materials. The shroud tip assembly is relatively heavy and therefore would limit the rotational speed of the rotor and blades to a lower speed than the present invention.
Thus, there is a need to improve the sealing between blades by including a tip shroud, and to increase the diameter of the blades in the last stage of the turbine in order to extract the greatest amount of power from the flowing gases while also improving the vibration damping characteristics of the shroud ring.